The prior methods for improving interlaminar peel strength in the production of multilayer printed wiring boards with an internal layer are comprised in the following:
(i) Method comprising the use of copper foil which has previously been made uneven;
(ii) Method comprising forming a printed wiring on a copper-clad laminate having a gloss surface, followed by treating the copper surface with an alkaline oxidizing solution, thereby forming a brown or black oxidized copper membrane; and
(iii) Method comprising forming a printed wiring on a copper-clad laminate having a gloss surface, followed by treating the copper surface with a silane coupling agent or an organic titanate coupling agent.
In order to improve these prior methods, a method comprising forming a printed circuit on a copper-clad laminate having a gloss surface, followed by treating the copper surface with an alkaline oxidizing solution, thereby forming a brown or black oxidized copper membrane has recently been used from view points of economy and adhesion properties. However, oxidized copper membrane, namely cupric oxide formed by this method is likely to be dissolved in an acidic aqueous solution, such as a hydrochloric acid solution or a sulfuric acid solution.
In general, multilayer printed wiring board was subjected to a treatment, such as, after drilling small holes, through-hole plating, electroless plating, or electroless plating, followed by electro-plating. When the above-mentioned prior method is applied to the printed board in order to improve the adhesion properties, the exposed oxidized copper in the through-holes tends to be dissolved by the acidic aqueous solution during the plating. This causes the formation of "Haloing" or "Pink Ring". These phenomena lower the reliability of the insulating printed circuit board.
A method which comprises treating the copper surface of the printed circuit board with an alkaline reducing aqueous solution which tends to be generated in the brown or black oxidized copper has been known for preventing "Haloing" or "Pink Ring" [refer to Japanese Patent Publication (KOKAI) No. 153797/1981]. Though this method exhibits adhesion properties of the copper surface which are available in practical operation, the effect for preventing "Haloing" depends on the case-by-case. Therefore, this method is not pratically used.